When a controlled drive system is referred to herein, we mean to indicate a drive system having an electric motor which can be provided with a feedback type of control. When a plurality of controlled drive systems are used to distribute a driving force onto a common element, as is the case for example in a metal strip processing line, the drives for individual rolls of a leveling machine, twin-drive roll stands, apparatus for fabricating or processing textile and paper and the like, to avoid an asymmetric load distribution upon the individual drive system and the detrimental effect this may have upon the drive systems and the workpiece, it is desirable to effect a load balancing.
For example, the asymmetric load distribution can be compensated by a control process in which the difference between the armature currents of two successive drives produces a signal which is used, in turn, to reduce the speed setpoint in the speed control circuit of the more strongly loaded drive to thereby bring about a load balancing. This form of load balancing is based upon a cascade regulation system, i.e. each pair of drives successively along the workpiece produces such a difference signal and that difference signal is used to reduce the speed setpoint of the more strongly loaded drive of the pair. The signal for load balancing must first be compared to an output from the slower speed controller and then supplied to the current controller. This has the drawback that the load-balancing system operates with relatively high inertia, i.e. cannot respond rapidly to load fluctuations. Such a load-balancing system is not suitable for machines with high dynamic requirements as is the case, for example, with metal strip leveling in a roller-type leveling machine wherein individual drives are provided for the leveling rollers.